Electro-optic display cells, such as electrochromic, electrophoretic or liquid crystal displays, employ a liquid electrolyte in a hermetically sealed enclosure. During construction, the liquid is introduced through one or more filling holes which must subsequently be hermetically sealed with, typically, a glass, resin, or solder plug. Although generally located in a faceplate of cells of sandwich type construction, filling holes are also known in the spacer, or side wall, separating two such plates. Typical of this prior art are U.S. Pat. Nos. 4,188,095, 4,199,228 and 4,239,350.
The use of elastomers in sealing arrangements for such cells is also known. Published European patent application EU 3145-A1 shows a liquid containing display of sandwich construction in which two filling holes are provided, one in each faceplate. A common elastomeric plug is drawn through both these holes to seal them. The plug has broad ends to prevent it working loose. An alternative arrangement with a single filling hole is also described in which a sealing plug is drawn through a channel shaped "brace" on the exterior of the faceplate so as to cover the filling hole.
U.S. Pat. No. 4,135,789 essentially describes the sealing of a filling hole with a glass plug which is fused into the hole by heating through a metal cap. Also provided below the glass plug is a polytetrafluoroethylene "separator" pad which is elastomeric and resiliently engages the side walls of the filling hole. The basic purpose of the pad is to isolate the underlying liquid crystal thermally during the fusing of the glass plug.
One problem encountered with such hermetically sealed liquid filled displays is that of differential thermal expansion between the liquid and its enclosure. Published British patent application No. 2,046,935A shows a sandwich cell construction in which a pair of glass plates are sealed around their edges by a double epoxy resin spacer and sealant, the inner portion of which is cured by reaction with the electrolyte. The same resin sealant is used to seal a filling hole. Such adhesive sealing techniques have the disadvantage that the electrolyte may become contaminated by the adhesive or a curing agent. This publication mentions that the peripheral sealant must have sufficient flexibility to withstand thermal expansion of the substrates and electrolyte. However, if the volume of liquid is at all large, such an expansion mechanism may be inadequate and a risk of rupturing the seal may exist.
An alternative approach to the expansion problem is shown in published European patent application EU 30493A involving the deliberate introduction of a gas bubble in an expansion chamber segregated from the main display chamber. A similar approach is shown in U.S. Pat. No. 4,310,220. It has been found that although the gas bubble cannot pass directly from the expansion to the display chamber, thermal cycling of the display can result in some gas dissolving and later reforming as bubbles in the display chamber.